Point at the Coffey-Labs organisation

The repositories moved off LINUXexpert-org. The old URLs redirect, so
nothing was broken, but a redirect is not a correct address to publish.

The SOURCE_URL defaults matter most: the AGPL asks whoever runs a
modified version to offer that version's source, and the sign-in page and
About screen show this link. It is in four places that have to agree --
the compose file, .env.example, the server default and the web fallback.

The rest is documentation and issue links.

server/src/config.ts

@@ -127,7 +127,7 @@ export const config = {
    * source, not the one it was forked from -- so anyone deploying a patched
    * ihasmail should point this at their own tree.
    */
-  sourceUrl: env("SOURCE_URL", "https://github.com/LINUXexpert-org/ihasmail"),
+  sourceUrl: env("SOURCE_URL", "https://github.com/Coffey-Labs/ihasmail"),
   host: env("HOST", "0.0.0.0"),
   port: int("PORT", 8080),
   stalwartUrl,